1. Field of the Invention
The present invention relates to a luminance signal automatic gain control (AGC) unit of a digital video decoder for decoding a video signal.
2. Description of the Related Art
Digital video decoders comprise an AGC circuitry in order to obtain a stable output even if the amplitude of an input signal is varied.
FIG. 1 is a block diagram of a conventional AGC circuitry.
As shown in the figure, the conventional AGC circuitry comprises an AGC preprocessing circuit 101 and an AGC circuit 102.
The AGC preprocessing circuit 101 receives a composite signal and a sync tip detecting signal to output a luminance signal Y.
FIG. 2 is a block diagram of a conventional AGC preprocessing circuit.
As shown in FIG. 2, the AGC preprocessing circuit includes an analog to digital conversion part 103, a Y/C separation part 104, a sync tip level detection part 105, and a sync tip level subtracter part 106. A functional overview for each part will be described with reference to the drawings.
FIGS. 3A to 3D are waveform diagram of a video signal illustrating the function of a conventional AGC preprocessing circuit.
FIG. 3A is a view explaining the function of the analog to digital conversion part 103 of FIG. 2. As shown in FIG. 3A, the analog to digital conversion part 103 receives an analog composite signal for A/D conversion and outputs a digital composite signal. The figure shows that an analog composite signal is converted into a digital composite signal scaled in 8 bits for 255 values. As shown in the figure, the amplitude of the digital composite signal A/D-converted is attenuated to narrower range than the range of 255 values. It is a role of a luminance signal AGC circuit to automatically amplify the amplitude to the range of 255 values.
FIG. 3B is a view explaining function of the Y/C separation part 104 of FIG. 2. As shown in FIG. 3B, the Y/C separation part 104 receives an 8-bit digital composite signal from the analog to digital conversion part 103. The Y/C separation part 104 then separates the received signal into a color signal and a luminance signal Y to output only an 8-bit luminance signal.
FIG. 3C is a view explaining function of the sync tip level detection part 105 of FIG. 2. As shown in FIG. 3C, the sync tip level detection part 105 detects the sync tip level of the 8-bit luminance signal Y received from the Y/C separation part 104 in matching with the timing of reception of the sync tip detecting signal from outside.
FIG. 3D is a view explaining function of the sync tip level subtracter part 106 of FIG. 2. As shown in FIG. 3D, the sync tip level subtracter part 106 receives the 8-bit luminance signal Y from the Y/C separation part 104, and subtracts the sync tip level which is detected by the sync tip level detection part 105 from the luminance signal Y. By subtracting the sync tip level from the 8-bit luminance signal Y, the 8-bit luminance signal Y is level-shifted as a whole so that the sync tip reaches the axis of zero.
Referring again to FIG. 1, the AGC circuit 102 includes a gain multiplying part 107, a pedestal level detection part 108, and a gain calculation part 109. A Functional overview for each part will be described with reference to the drawings.
The gain multiplying part 107 receives the luminance signal Y from the AGC preprocessing circuit 101. The gain multiplying part 107 then amplifies the amplitude of the luminance signal Yα1-fold to output the resultant luminance signal Yα1.
The pedestal level detection part 108 receives a pedestal level detecting signal to detect a pedestal level of the luminance signal Yα1.
FIG. 4 is a diagram explaining function of a conventional pedestal level detection part.
As shown in the figure, the pedestal level detection part 108 calculates the average of the pedestal level of the luminance signal Yα1 corresponding N pixels before a pedestal level detecting signal is received in matching with the reception of the pedestal level detection signal. The averaged pedestal level is then output as the pedestal level of the luminance signal Yα1, which represents a synchronizing level, since the sync tip level of the pedestal level is shifted to the value of zero in 8 bits (FIG. 3D).
The gain calculation part 109 receives the averaged value of the pedestal level from the pedestal level detection part 108 and estimates an amplitude level of the luminance signal Yα1. From the estimation result, the gain calculation part 109 calculates an amplification factor α2, and transmits it to the gain multiplying part 107. The explanation of the form of a video signal based on the IRE reference UNIT is explained here, though in the course of the explanation of the gain calculation part 109.
FIGS. 5A and 5B are schematic diagrams explaining a video signal.
FIG. 5A is a view explaining the change of a video signal in a vertical blanking period and an effective video period. FIG. 5B is a view explaining the form of a video signal.
In the video signal as shown in FIG. 5B, the pedestal level is set to 0 IRE, the picture level is in the plus direction, and the synchronizing level is in the minus direction. The white level goes to approximately 100 IRE and the sync tip level goes to approximately −40 IRE. Since the sync tip level is approximately −40 IRE, the synchronizing level is 40 IRE, which equals to value of 64 in 8 bits.
Referring again to FIG. 1, the gain calculation part 109 receives the pedestal level of the luminance signal Yα1 from the pedestal level detection part 108. The gain calculation part 109 then compares the pedestal level with a comparison reference of the value of 40 IRE. In the conventional art, the value of 40 IRE is held in the gain calculation part 109. From the comparison result, the gain calculation part 109 calculates the amplification factor α2 which is used to amplify the amplitude of the luminance signal Yα1 to the level shown in FIG. 5B and transmits the factor α2 to the gain calculation part. It should be noted here that the pedestal level of the luminance signal Yα1 received from the pedestal level detection part 108 represents the synchronizing level of the luminance signal Yα1, and that the comparison reference 40 IRE represents the magnitude of the synchronizing signal in a standard video signal.
In other words, the gain calculation part 109 receives the synchronizing level of the luminance signal Yα1 and calculates the amplification factor to make the synchronizing level match the synchronizing level of a standard video signal. As a result of the multiplication of the luminance signal Yα1 by the amplification factor α2 determined in this way, the synchronizing level of the luminance signal Yα1α2 is equalized to the synchronizing level of the standard video signal. In addition, the video level of the luminance signal Yα1α2 is also equalized to the video level of the standard video signal since the ratio between the video level and the synchronizing level stay constant.
In this process, if the luminance signal Yα1 is just multiplied by the amplification factor α2 in the gain multiplying part 107, a rapid change may occur and a screen may be affected. Thus, the amplification factor α2 is usually disassembled into the form of α2=β1·β2 . . . βn for amplifying the luminance signal Yα1 to the luminance signal Yα1α2 step by step at n times.
In recent years, a variety of types of video equipment have been developed. Those video equipment types sometimes use video signals of which sync tip level is different from −40 IRE (e.g. −30 IRE). In a prior art, whenever the sync tip level of the video signal used in an video equipment is different from −40 IRE, a gain offset set in the gain calculation part 109 should be manually adjusted. However, the number of types of video equipment have been rapidly increased. Recently, there is video equipment using different values of sync tip levels between the vertical blanking period and the effective video period, for example, −30 IRE in the effective video period, while −40 IRE in the vertical blanking period. The conventional technique to manually adjust them cannot cope with such a situation.
The prior art is disclosed in Japanese Patent Kokai No. 8-322003 (patent document 1).
In the prior art, whenever video apparatus applies a video signal having a sync tip level different from a standard one, that is −40 IRE, a reference for comparison set in a gain calculation part of a video equipment should be manually adjusted. A problem has been encountered that it is difficult to manually adjust the video apparatus of which sync tip levels in an effective video period and a vertical blanking period are different from each other.